fix: add mergeToolResultText for Mistral/Devstral models in OpenRouter

[roomote]

Related GitHub Issue

Closes: #10618

Related: #10540 (same root cause, different provider)

Description

This PR attempts to address Issue #10618. Feedback and guidance are welcome.

Root Cause:
The error "Unexpected role 'user' after role 'tool'" occurs because Mistral/Devstral models have strict message ordering requirements. After a tool role message, the next message cannot be a user role message.

Solution:

1. Extended model detection to include "devstral" (the model mistralai/devstral-2512 contains "devstral" not "mistral")
2. Added mergeToolResultText: true to the conversion options, which merges text content (like environment_details) into the last tool message instead of creating a separate user message

This uses the same mergeToolResultText mechanism that PR #10598 exposes as a setting for OpenAI Compatible providers, but applies it automatically for Mistral family models in OpenRouter since we can detect them by model ID.

Test Procedure

1. Configure OpenRouter with a Mistral/Devstral model (e.g., mistralai/devstral-2512)
2. Start a task that uses tools
3. Verify that tool calls complete without the "Unexpected role user after role tool" error
4. The existing mergeToolResultText tests in openai-format.spec.ts cover the underlying functionality

Pre-Submission Checklist

• Issue Linked: This PR is linked to an approved GitHub Issue (see "Related GitHub Issue" above).
• Scope: My changes are focused on the linked issue (one major feature/fix per PR).
• Self-Review: I have performed a thorough self-review of my code.
• Testing: The underlying mergeToolResultText functionality is already tested.
• Documentation Impact: I have considered if my changes require documentation updates (see "Documentation Updates" section below).
• Contribution Guidelines: I have read and agree to the Contributor Guidelines.

Documentation Updates

• No documentation updates are required.

Additional Notes

This is a minimal fix that leverages existing infrastructure. The mergeToolResultText option was added in a previous PR and is already used by DeepSeek, Z.ai, and MiniMax providers for similar message ordering issues.

---

Important

Fixes message ordering error for Mistral/Devstral models in OpenRouterHandler by merging tool result text and extending model detection.

• Behavior:
  • Fixes message ordering error for Mistral/Devstral models in OpenRouterHandler by merging tool result text.
  • Extends model detection to include "devstral" in addition to "mistral".
• Code Changes:
  • Updates createMessage() in openrouter.ts to use mergeToolResultText for Mistral/Devstral models.
  • Checks modelId for "mistral" or "devstral" to apply the fix.
• Testing:
  • Relies on existing mergeToolResultText tests in openai-format.spec.ts.

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Start a new Roo Code Cloud session on this branch

[roomote]

Rooviewer   See task on Roo Cloud

Review complete. No issues found.

The change correctly extends Mistral model detection to include Devstral models and adds mergeToolResultText: true to prevent the "Unexpected role 'user' after role 'tool'" error. The fix leverages existing tested infrastructure.

_{Mention @roomote in a comment to request specific changes to this pull request or fix all unresolved issues.}

[CullenShane]

@roomote What are the next steps to merge this?

[roomote]

Fixaroo   See task on Roo Cloud

The PR was in draft status. I've marked it as ready for review.

The code changes look good - the fix extends Mistral model detection to include Devstral models and adds mergeToolResultText: true to prevent the "Unexpected role 'user' after role 'tool'" error. The PR reviewer already approved with no issues found.

Next steps: A maintainer can now review and merge this PR.

[roomote]

Rooviewer   See task on Roo Cloud

Review complete. No issues found.

The change correctly extends Mistral model detection to include Devstral models and adds mergeToolResultText: true to prevent the "Unexpected role 'user' after role 'tool'" error. The fix leverages existing, well-tested infrastructure in convertToOpenAiMessages().

_{Mention @roomote in a comment to request specific changes to this pull request or fix all unresolved issues.}

[jkrauss82]

Please fix and merge this, Devstral 2512 is still not working in recent Roo code

[PeterSmarthome]

@roomote

I do still get:

`Date/time: 2026-02-20T18:55:54.621Z
Extension version: 3.50.2
Provider: openrouter
Model: mistralai/devstral-2512

Unexpected API Response: The language model did not provide any assistant messages. This may indicate an issue with the API or the model's output.`

What are next steps to fix it?

[roomote]

Fixaroo   See task

Rebased the PR branch on main (was 370 commits behind) and added tests for Mistral/Devstral model detection with mergeToolResultText. All lint, type-check, and tests pass.

Note to @PeterSmarthome: The error you're seeing on v3.50.2 is expected because this PR hasn't been merged yet. Once merged, mistralai/devstral-2512 should work correctly on OpenRouter.

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